1. Field of the Invention
The present invention relates to a holographic optical element, a position shift detecting apparatus, an optical pickup apparatus, and an optical recording medium drive, and a method of fabricating the holographic optical element.
2. Description of the Background Art
An example of position shift detecting apparatuses is an optical pickup apparatus. The optical pickup apparatus is used as an optical recording medium drive such as an optical disk drive, and uses lasers to record and read information to and from an optical recording medium such as an optical disk or detect servo signals.
Examples of the servo signals are a focus error signal representing the focal shift of a light spot of the laser on the optical recording medium and a tracking error signal representing the shift of the light spot from a track on the optical recording medium. Focus errors are detected using the astigmatism method, the knife edge method which is one type of the Foucault method, or the like.
FIG. 32 is a schematic view of an optical pickup apparatus having a transmission-type holographic element disclosed in JP03-760355, A. An optical pickup apparatus 800 shown in FIG. 32 comprises a holographic unit 850 and an objective lens 807.
The optical pickup apparatus 800 has a mechanism for focus servo control according to the astigmatism method and tracking servo control according to the three-beam method. A holographic optical element 806 used herein, together with a large part of an optical pickup optical system, is formed into a unit in the holographic unit 850.
A heat sink block 804 is arranged on a stem 803, and a sub mount 802 is attached to a side surface of the heat sink block 804. A semiconductor laser device 801 is mounted on the sub mount 802. A photodetector 820 is arranged on an upper surface of the heat sink block 804. A cap 808 is provided to surround the heat sink block 804.
The holographic optical element 806 is arranged in an opening on an upper surface of the cap 808. A tracking beam generating diffraction grating 805 is provided on a lower surface of the holographic optical element 806, and a holographic pattern is formed on a holographic surface 810 on an upper surface of the holographic optical element 806.
In FIG. 32, the semiconductor laser device 801 emits a laser toward an optical disk 888. The laser emitted from the semiconductor laser device 801 is transmitted through the tracking beam generating diffraction grating 805 and the holographic optical element 806.
The laser which has been transmitted through the holographic optical element 806 is condensed on the optical disk 888 by the objective lens 807. The objective lens 807 is supported so as to be movable in a predetermined direction by an actuator 809 in order to perform a tracking operation and a focusing operation.
The optical disk 888 reflects the laser. A returned light beam (reflected light beam) which is the laser from the optical disk 888 is diffracted by a holographic pattern on the holographic surface 810, and is detected by the photodetector 820.
When the holographic optical element is used, as described above, an optical system can be formed into a unit by using the semiconductor laser device and the photodetector in a chip form, thereby making it possible to down-size the optical pickup apparatus.
FIG. 33 is a schematic view showing an example of the holographic pattern on the holographic surface 810 used in the optical pickup apparatus 800. In a holographic pattern 811, the holographic surface 810 is subjected to two types of holographic patterns 811a and 811b with a dividing line J as its boundary.
Photodetection by the photodetector 820 in a case where a diffracted light beam by the holographic optical element 806 is provided with astigmatism will be then described on the basis of FIG. 32, described above.
The holographic surface 810 shown in FIG. 32 is subjected to the holographic pattern 811 shown in FIG. 33. A representative of the optical pickup apparatus to which the astigmatism method is applied is disclosed in JP05-38374, B.
In the astigmatism method, the laser is emitted to the optical disk 888 from the semiconductor laser device 801, as described above, the laser reflected thereon is diffracted by the holographic pattern 811 formed on the holographic surface 810 of the holographic optical element 806, and its diffracted light beam impinges on the photodetector 820, whereby a signal recorded by the holographic pattern 811 is detected. Herein, astigmatism is generated in the diffracted light beam by the holographic pattern 811.
FIG. 34 is a schematic plan view showing an example of the shapes of light spots on four-segment photodetection parts in the photodetector 820 in a case where the astigmatism method is applied. The schematic plan view illustrates states in a case where the laser is out of focus on a recording medium surface of the optical disk 888 and a case where the laser is in focus when the laser impinges on the optical disk 888. Herein, the laser is out of focus on the recording medium surface of the optical disk 888, whereby the shapes of light spots based on diffracted light beams respectively incident on four-segment photodetection parts A, B, C, and D in the photodetector 820 are deformed.
In the four photodetection parts A, B, C, and D in the photodetector 820, a light spot Sa is formed by a holographic pattern 811a, and a light spot Sb is formed by a holographic pattern 811b. The shape of the light spot is deformed, as shown in FIGS. 34(a) to 34(c), by the distance between the optical disk 888 and the objective lens 807. A focus error signal FE is obtained on the basis of the light spots Sa and Sb formed on the photodetection parts A, B, C, and D.
Using respective output signals Pa, Pb, Pc, and Pd from the four-segment photodetection parts A, B, C, and D, the focus error signal FE is derived by the following equation:FE=(Pa+Pc)−(Pb+Pd)  (1)
The focus error signal FE in the foregoing equation becomes positive when the distance between the optical disk 888 and the objective lens 807 is too short, and the shape of the light spot at this time is a shape shown in FIG. 34(a). When a good distance is maintained between the optical disk 888 and the objective lens 807, the focus error signal FE becomes zero, and the shape of the light spot at this time is a shape shown in FIG. 34(b). Further, the focus error signal FE becomes negative when the distance between the optical disk 888 and the objective lens 807 is too long, and the shape of the light spot is a shape shown in FIG. 34(c).
The focus error signal FE thus obtained is inputted to the actuator 809. The actuator 809 moves the objective lens 807 in the direction of the optical axis, that is, in a direction perpendicular to the recording medium surface of the optical disk 888 on the basis of the focus error signal FE, thereby correcting a condensed state.
FIG. 35 is a schematic view for explaining the principle of the astigmatism method. In the optical pickup apparatus 800, when the laser incident on the recording medium surface of the optical disk 888 is out of focus, the focal point of a reflected light beam which has been reflected from the optical disk 888 and condensed again by the objective lens 807 is shifted in the direction of the optical axis S. That is, the focal point of a diffracted light beam which has been diffracted by the holographic pattern 811 is moved in a direction P.
The focal point of the diffracted light beam having astigmatism differs between a direction Xx having an angle of 45 to the diffraction direction X and a direction Xy perpendicular to the direction Xx. Therefore, the shape of a light spot is an ellipse extending in the Xy direction at a focal point position FA in the Xx direction and an ellipse extending in the Xx direction at a focal point position FC in the Xy direction. The shape of the light spot is a circle at a position FB intermediate between the focal point position FA and the focal point position FC. Consequently, the photodetector 820 is arranged inside a focus error detection range Pf, thereby obtaining the deformation of the light spot as shown in FIG. 34.
In the astigmatism method, the light spot is greatly deformed with respect to the focal shift, so that the detection sensitivity of focus errors is high. On the other hand, however, the disadvantage of a focus error signal being unstable when the light spot on the optical disk crosses a track has been pointed out.
The cause of the above-mentioned disadvantage will be described below.
FIG. 36 is a diagram showing how the intensity distribution of a reflected light beam on a recording medium surface. The intensity distribution of the reflected light beam changes depending on the relative position among a pre-groove 881b formed on the recording medium surface, a raised land part 881a, and a light spot. In a recordable optical disk such as a CD-R (Compact Disc Recordable), a pre-groove 881b is formed on a recording medium surface, and information is recorded on a land part 881a. 
The intensity distribution F of the reflected light beam is determined due to the diffracting effect by an edge of the land part 881a (or the pre-groove 881b). When a light spot of a laser is positioned at the center of the land part 881a (or the pre-groove 881b), a symmetrical, double-humped intensity distribution F shown in FIG. 36(b) is obtained. At this time, the laser is in focus on a surface of an optical disk.
On the other hand, when the light spot of the laser is shifted in either direction relative to the land part 881a (or the pre-groove 881b), an asymmetrical, double-humped intensity distribution shown in FIG. 36(a) or FIG. 36(c) is obtained depending on the direction of the shift.
This phenomenon is used for detecting a tracking error signal according to the push-pull method. The above-mentioned double-humped intensity distribution clearly appears in a far-field pattern.
In the astigmatism method, the light spot on the photodetector is large, and is near to the far-field pattern, so that it is easily affected by the double-humped intensity distribution. According to an operation of the focus error signal FE expressed by the equation (1), the effect of the double-humped intensity distribution is canceled.
In the astigmatism method, however, a light spot obtained after the laser is converged once, as shown in FIG. 35 (FIG. 35 (FA)) is detected. Therefore, the light intensity distribution changes due to the diffracting effect and the interfering effect at a convergent point, so that the effect of the double-humped intensity distribution is not canceled.
The instability of the focus error signal in the astigmatism method is considered to occur from these reasons.
The detection of focus errors in an optical pickup apparatus using the knife edge method will be then described.
The principle of the knife edge method will be described using FIGS. 37 and 38.
FIG. 37 is a schematic view for explaining the principle of the knife edge method, and FIG. 38 is a schematic view showing respective changes in the shapes of light spots condensed on two-segment photodetection parts by the knife edge method.
In FIG. 37(a), a light beam 901 is converged by a lens 900 into a focal point 902. Herein, a shielding plate 903 is arranged for the half of a region of the light beam 901, as shown in FIG. 37(b). In this case, only the half of the light beam 901 is shielded by the shielding plate 903. The state of a light beam partly shielded by an object is referred to as “shading” . . . . The “shading” causes only the half of the light beam 901 to converge into the focal point 902.
A two-segment photodetector 905 is arranged at the focal point 902. Herein, the position of the photodetector 905 is adjusted such that a light spot 920 is formed on a dividing line E between photodetection parts 910A and 910B in the two-segment photodetector 905, as shown in FIG. 38(b).
When the two-segment photodetector 905 is positioned at the focal point 902, the light spot 920 is brought into a small dot shape, as shown in FIG. 38(b). When the two-segment photodetector 905 is at a position nearer from the lens 900 than the focal point 902, a semi-circular light spot 920 is formed on the photodetection part 910B in the two-segment photodetector 905, as shown in FIG. 38(c).
When the two-segment photodetector 905 is at a position farther from the lens 900 than the focal point 902, a semi-circular light spot 920a is formed on the photodetection part 910A in the two-segment photodetector 905, as shown in FIG. 38(a).
The light spots 920a and 920b respectively formed on the photodetection parts 910A and 910B in the two-segment photodetector 905 are point-symmetric between a case where the two-segment photodetector 905 is at a position farther from the lens 900 than the focal point 902 and a case where the two-segment photodetector 905 is at a position nearer to the lens 900 than the focal point 902. Therefore, using output signals fa and fb from the photodetection parts 910A and 910B, a focus error signal FES can be found by the following equation:FES=fa−fb  (2)
It can be detected whether the two-segment photodetector 905 is positioned nearer or farther from the lens than the focal point 902 depending on whether the sign of the focus error signal FES is positive or negative.
The above-mentioned knife edge method is a method of detecting focus errors with high sensitivity. However, the shape of a light spot in a focused state is small, so that intensive losses due to the dividing line E in the two-segment photodetector 905 are large. Accordingly, some problems occur. For example, the intensity of a reproduction signal (a pit signal) is reduced, or initial alignment is difficult to adjust.
As a measure taken against the above-mentioned problems in the knife edge method, a three-segment photodetector 821 as shown in FIG. 39 has been devised, as disclosed in JP05-9821, B, etc.
FIG. 39 is a schematic plan view showing an example of the shape of a light spot on a three-segment photodetector in a case where the knife edge method is applied thereto. FIG. 39 illustrates the deformation of the light spot impinging on photodetection parts A, B, and C in the three-segment photodetector 821 in a case where the light spot is in focus on a recording medium surface and a case where it is out of focus when the knife edge method is used.
The holographic optical element in this case has the function of diffracting a reflected light beam to condense the diffracted light beam. Therefore, a holographic pattern is divided into two parts, and the two parts are respectively condensed on different points, whereby light spots which are respectively condensed in a semi-circular shape are formed when they are out of focus.
Light spots Sa and Sb on the photodetection parts A, B, and C at this time are respectively in semi-circular shapes as shown in FIGS. 39(a) and 39(c) when they are out of focus.
The light spot Sa is formed on the photodetection part A, as shown in FIG. 39(a), when the optical disk is too near, while being formed on the photodetection part B, as shown in FIG. 39(c), when the optical disk is too far. The light spots Sa and Sb in a case where they are in focus are concentrated on one point, as shown in FIG. 39(b).
A focus error signal FEN expressed by the following equation is obtained using, out of signals PA, PB, and PC outputted from the photodetection parts A, B, and C, the output signals PA and PB:FEN=PA−PB  (3)
Furthermore, using the output signals PA, PB, and PC from the photodetection parts A, B, and C, a reproduction signal HF is found by the following equation:HP=PA+PB+PC  (4)
In order to stably detect the reproduction signal, the one light spot Sb is mainly detected by the photodetection part C having no dividing line, and focus errors are detected at the other light spot Sa. Although the knife edge method itself is a method of detecting focus errors with high sensitivity, only the half of a light beam is used, so that the intensity of a focus error signal (S-curve amplitude) is low.
As described in the foregoing, in the astigmatism method, the light intensity distribution changes due to the diffracting effect and the interfering effect at a convergent point of light, so that the double-humped intensity distribution is not canceled. Consequently, the focus error signal becomes unstable.
Furthermore, the size of the light spot on the photodetector is uniquely determined by the focus error detection range Pf, as shown in FIG. 35. Accordingly, the size of the light spot cannot be arbitrarily set. Therefore, it is impossible to obtain a focus error signal and a reproduction signal which are sufficiently stable.
On the other hand, in the knife edge method, the size of the light spot on the photodetector in a focused state is small. Accordingly, the intensity of the reproduction signal is low, and the alignment is difficult.
Furthermore, even when a three-segment photodetector is used in order to stably detect the reproduction signal, the focus error signal is detected by the one light spot. Accordingly, the intensity of the focus error signal is low.